1. Field of the Invention
The present invention relates to an article and process for producing electronic devices. More particularly the invention relates to an article and process for fabricating patterns of electrically conductive materials on non-electrically conductive substrates by laser ablation. Such an article and process find use in the production of RFID devices, antennae, electrical circuits, microwave susceptors, contacts, leads, conductors, sensors, interactive displays, electrostatic shielding devices, heating elements and the like.
2. Description of the Related Art
There is great commercial interest in a new generation of data transmitting electronic devices such as transponders, sensors, etc. The use of electronic components in everyday items, particularly in packaging, in the form of traceable tags, heating elements, security elements, sensors etc, is growing rapidly.
Recently, Radio Frequency Identification Devices or RFID's have found use in providing tagging and traceability for the packaging industry. RFID refers to technologies that use radio waves to identify and transmit information remotely from tagged objects. In a typical RFID tag, a microchip is attached to an antenna. The antenna is typically made of conductive metal, e.g., silver, copper, etc., and can have various shapes and geometries based on the radio frequency used. The chip and the antenna together constitute an RFID transponder or a tag. The chip contains the necessary information for identifying a tagged item. The function of the antenna is to communicate remotely through radio wave signals from a reader and establish the identity of the object. The ability to remotely monitor the flow of items from one place to another makes RFID an attractive technology. Its use in inventory control, security, etc. for packaged goods has serious financial ramifications. The ultimate goal is to implement an item level tagging. However, there are several roadblocks to the commercial use of RFID technology. A key factor is the inability to mass produce tags with required functionalities in a simple, reliable, and consistent manner, and producing them cost effectively, for making widespread applications feasible. At present, the cost of RFID transponders ranges from 50 cents to several dollars each, depending on their functional requirements. However, for item level tagging to be feasible, which would require billions of tags, the cost of the transponders has to be significantly less. Unfortunately, the currently available methods of tag manufacturing are not capable of meeting these requirements and therefore, the industry and the market, are at a stand still. The cost of a transponder is primarily dictated by (1) the cost of the antenna (2) the cost of the base material used (3) the cost of the chip (4) the cost of fabrication (5) the cost of quality control (6) the final yield value and (7) the overall capital investment required. The cost of the antennae has been identified to be one of the key limiting variables and therefore, an intense effort is being made to overcome this problem. Currently a method of chemical etching is used to produce an antenna from a conductive metal base. The process involves selective chemical etching of a conductive metal like silver or copper, with a strong acid or base to produce antennae of selective geometries. In subsequent multiple steps the antenna is attached to a chip via a conductive adhesive and is encased in an appropriate casing. The process is old, well understood and broadly practiced, however, wasteful, environmentally harmful, expensive, and therefore it is not able to take the industry to the next level. Several other methods e.g., metal stamping, selective electrolytic metal deposition, etc. have been tried as alternatives. However, none has been successful as a solution.
It is known in the art to produce electronic devices such as antennae by a technique generally described as “plating on plastics” The “plating on plastics” technology deposits an adherent coating of a metal or metal-based material onto the surface of a plastic substrate. “Plating on plastic” envisions the deposition of an initial metal coating using “electroless” plating followed by an optional deposition of metal using electrodeposition. Electroless plating involves chemically coating a nonconductive surface such as a plastic with a continuous metallic film. In this regard, U.S. Patent Application 20020135519 describes a radio frequency identification device (RFID) and many different ways of manufacturing such RFID antennae and transponders. An RFID transponder device is essentially composed of a small antenna attached to a programmable computer chip. Such transponders are required for transmitting electromagnetic radiation between the device and its surroundings, e.g., a reader or negotiator. Together, a transponder and a reader constitute a working RFID system. An RFID transponder may be passive, which can be used for transmission short range transmission. However, they may also be active and used for longer range transmission when a battery is attached to the transponder. Typically an RFID antenna is a flat metallic structure comprising of conductive and non-conductive areas. Its appropriate geometrical form is dictated by the frequencies used by the readers, typically in the kilohertz to megahertz ranges. An active or passive transponder is typically encased in a plastic laminated structure, e.g., a label, which is then attached to a pallet, case or boxes to obtain non-contact and remote “tag and traceability” features for inventory control, logistics, security and various other functions. However, most of the solid state electronic circuitry is rigid and therefore, not suitable for use on packaging items, as most such items are flexible materials. Therefore, a tremendous amount of effort is being used developing so called “printed electronics.” The goal of printed electronics is to print the required elements of an electrical circuitry onto the flexible substrate itself. The printed substrate with required elements can then be put onto the finished packaging as an added label, or as part of the packaging itself, or the item itself, thereby imparting the appropriate functionalities to the packaging and packaged items.
An essential element of an electronic circuit is geometric patterns of conductive and nonconductive (dielectric) areas. Typically this is done in a semiconductor circuit via the doped metal layers and dielectric layers. In a printed circuit, printing inks made with conductive particles, typically silver, are printed in respective patterns on a nonconductive (dielectric) film surface. Majority of commercially manufactured antennas today are based on selective chemical etching of metals in appropriate geometries. However, this process poses serious environmental and health and safety hazards.
Recently printing of RFID antennae with conductive inks is being developed as an environmentally friendly approach. Typically, inks containing expensive metals like silver powder, are printed onto dielectric substrates, e.g., plastic or paper, forming a conductive pattern. After printing, the printed structure requires a high temperature annealing process to sinter all the conductive particles together and form continuous conductive elements, in order to achieve necessary levels of conductivity. This limits the number of substrates that can be used to print on, since only expensive substrates with high softening temperature may be used to avoid any deformation. High temperature requirement also seriously reduces the overall productivity of the process. In addition, certain amount of ink film thickness must be deposited in order to obtain required conductivity levels. Moreover, due to high loading of silver powder requirement, the ink formulations must contain extraneous additives, including volatile organic compounds (VOC), to produce inks in a printable fashion. This makes the inks either less conductive or environmentally unfriendly. Printing of a substrate surface with a metal catalyst or “seed layer” and subsequent electrodeposition of copper metal in a bath onto the printed areas is another approach that is being studied. Although this process avoids any high temperature processing, it is a slow and wet process, with their inherent disadvantages.
German Patent DE 19951721 teaches a process of selective ablation of a thin, less than 250 nm metal layer, with UV lasers emitting between 248 nm and 532 nm. After ablation an additional step of metal electrodeposition is required in order to achieve required electrical conductivity. While this approach promises to yield silver free, less expensive antennae, the productivity of such process is very low since UV laser irradiation has a limited energy level for ablating thicker metal layers and electrodeposition process is extremely slow and requires utilization of wet chemistry.
All of these prior approaches also require subsequent multiple steps to convert the formed antennae into actual functional transponders. For example, a programmable computer chip must be attached to the antenna in a precise manner to obtain appropriate connection to the circuitry. Lack of connection would further reduce the yield of the process and above all, reliability of the transponders itself. For transponders with longer ranges, a battery must also be attached in a subsequent step.
Overall, the limitations of all the current and available approaches make the RFID antennae and transponders produced expensive, and limit their viability as commercially successful processes to satisfy projected industry needs for billions of inexpensive RFID transponders, for all types of applications including tagging of individual items. Similarly, commercial manufacturing of microwave susceptors or sensors also face similar limitations. These industries require a new, efficient, effective, environmentally friendly, robust, economical, and reliable manufacturing process to achieve their projected growth potentials.
At present the industry is focused on developing a printing method as a solution to this quandary. An ink made with conductive metal powder is printed on a flexible web substrate in the form of an antenna pattern. Next the printed patterns are heated to a high temperature, e.g., 130° C., to anneal the metal particles and form a continuous and conductive pathway. Subsequently, a suitable chip is attached in-line to the antenna via a conductive adhesive; quality controlled for right contacts and functionality, and the structure encased in an appropriate package for final assembly. However, this approach is complex. In order to achieve a high enough conductivity at printed film thickness, mainly silver is used as metal, which is expensive. The metal must also be in powder form with particle size ranging from micrometer to nanometer with their associated processing cost, further increasing the material cost. The substrate must be able to withstand high bake temperature ˜130° C. without thermal distortion, and therefore can primarily be expensive polyesters with high glass transition temperature (Tg). High web surface temperature requirement also slows down the overall printing process, increasing cost.
In order to convert the metal powder to an ink composition, resins, solvents, additives etc. must be added, most of which being non-conductive are not desired to remain with the printed antennae. Therefore, materials are selected which would volatilize during the baking step, which in turn create undesired environmental (VOC) and health & safety issues. In addition, despite the high heating step, the metal particles in the printed antenna simply do not attain their original metallic crystal structure and therefore results in printed antennae with somewhat reduced metal conductivity compared to their original value.
Attachment of computer chips onto the printed antennae is also a critical step. The chip connectors must be properly aligned right onto the antenna pads for appropriate coupling. Any misalignment would increase waste factor and above all would increase unreliability of the system, increasing potential risks and security breaches in several sensitive application areas. In order to minimize such risk factors “straps,” which are webs containing pre-positioned chips, can be positioned and aligned in a film lamination arrangement and chips transferred from the “strap” to the printed antenna web in-line. However, even with this approach there is at least 1-3% unreliability factor, which for mass adoption poses a great disadvantage.
The overall complexity, unreliability and economics of the current “printed antenna” process simply does not lend itself to be a solution to preventing the RFID market from achieving intended commercial success and there is a serious need for a new fabrication method for RFID antenna. The present invention discloses such an RFID antenna manufacturing process. The invented process starts with a metallized film which is provided with several computer chips in any pre-determined pattern via a conductive adhesive. This step can be done in-line or off-line. Next, an infrared (IR) laser ablates a portion of the metal layer in a predetermined manner to create an appropriate antenna form. A CAD/CAM or similar computer-hardware interface software can direct this exposure step. The process offers an increased in creating almost any possible antenna shape or form in one step. Since there is no baking step involved, an expensive high Tg film is no longer necessary. The metal layer can be almost any thin conductive metal layer, e.g., aluminum, silver, copper etc. However, a metallized Al layer is the most economical. The thickness of the Al layer can be adjusted easily to obtain a desired level of conductivity.
The present invention for mass manufacturing RFID devices avoids the roadblocks faced by the printing approach. The process does not require an expensive substrate, expensive conductive inks, no heat is required, no wet chemistry is involved and does not produce any VOC and/or health and safety issues. The process does not require a separate step of attaching computer chips to the formed antenna, thereby significantly improving the reliability, yield and robustness of the manufacturing process. The individual RFID devices, i.e., an antenna attached to a chip can be packaged in-line to produce a finished RFID tag in one single step.
Geometric patterns produced may be “passive,” e.g., an antenna for radio frequency identification device (RFID); a microwave susceptor; a circuit etc. They can also be “active” or “functional,” e.g., a complete transponder for RFID or a sensor or similar devices, when a metallized substrate, pre-fabricated with computer chip and a battery, is ablated. Any geometric form on a flat surface can be produced in a sheet or roll form. The conductive articles can be folded into a three dimensional architecture. Such production of fully active or passive devices, with high reliability and in high yield, produced in one step, is unique.